Decaying Tau Leptons Research Articles

Measurements of tau hadronic branching ratios in DELPHI experiment

This paper presents results on the measurement of tau-lepton hadronic decays branching ratios in DELPHI experiments at LEP. It includes the final results on exclusive hadronic branchings (without π / K separation) and preliminary results on single-prong tau-lepton decays to kaons.

TAUOLA as tau Monte Carlo for future applications

The status of the Monte Carlo programs for the simulation of tau-lepton production and decay in high-energy accelerator experiments is reviewed. In particular, the status of the following packages is discussed: (i) TAUOLA for tau-lepton decay, (ii) PHOTOS for radiative corrections in decays, (iii) MC-TESTER packages for various types of semi-automatic tests, and (iv) universal interface of TAUOLA for the decay of tau leptons produced by `any' generator. Emphasis is put on recent developments for high-precision tests and extensions of PHOTOS. Some considerations for the software organization necessary in future applications for Belle and BaBar will be given; examples of TAUOLA universal interface use will only be listed at the end of the review.

Microscopic black hole detection in UHECR: the double bang signature

According to recent conjectures on the existence of large extra dimensions in our universe, black holes could be produced during the interaction of Ultra High Energy Cosmic Rays with the atmosphere. However, and so far, the proposed signatures are based on statistical effects, not allowing identification on an event by event basis, and may lead to large uncertainties. In this note, events with a double bang topology, where the production and instantaneous decay of a microscopic black hole (first bang) is followed, at a measurable distance, by the decay of an energetic tau lepton (second bang) are proposed as an almost background free signature. The characteristics of these events and the capability of large cosmic ray experiments to detect them are discussed.

Selection of tau leptons with the CDF Run 2 trigger system

We have implemented triggers for hadronically decaying tau leptons within a framework of the CDF Run 2 trigger system. We describe the triggers, along with their physics motivations, and report on their initial performance.

Production and polarization effects in some tau-lepton decays

The conditions for the independence of decays of the spin- 1 2 resonances on the production mechanism of the resonances and on polarizations of the incoming and outgoing particles are derived and applied in the case of several tau-lepton decays. The necessity for inclusion of the influence of the production mechanism in the evaluation of the lepton flavour violating decays is stressed.

Spectral moments of two-point correlators in perturbation theory and beyond

We discuss the choice of weight functions for the moments of the spectral density for two-point correlators of hadronic currents over a finite energy interval. Of phenomenological relevance is an analysis of the spectra of tau lepton decay on the energy interval [0,M_tau] and the low-energy hadron production in e+ e- annihilation. General arguments are given for the calculability of such moments in perturbation theory using both a finite-order analysis and infinite resummation on the contour. Nonperturbative contributions emerging from the operator product expansion for two-point correlators are discussed within explicit models for the physical spectra. The quantitative analysis strongly disfavours weight functions that suppress the high-energy contribution to theoretical moments. This is in agreement with expectations from qualitative considerations in perturbative QCD with asymptotic freedom. We discuss the implication of our results for the ultimate accuracy that can be reached in tau decays and low-energy e+ e- annihilation into hadrons with present experimental data.

Implications of the muon anomalous magnetic moment and Higgs-mediated flavor changing neutral currents

In the light of the recent measurement of the muon anmalous magnetic moment $a_\mu$ by the Muon $(g-2)$ Collaboration, we examine the contribution to $a_\mu$ from the exchange of flavor changing scalars. Assuming that the heavier generations have larger flavor changing couplings, we obtain a bound on $\mu - \tau$ Yukawa coupling for a given scalar mass. Constraints on other flavor changing/conserving couplings are also obtained from the lepton flavor violating decays of muon and tau lepton, and bounds on the branching ratio of $\tau \to 3 e$, $\mu \to 3 e$ and $\tau \to e \mu e$ processes are predicted.

Search forCPviolation inτ→ππ0ντdecay

We search for CP non-conservation in the decays of tau leptons produced via $e^+ e^-$ annihilation at $\sqrt{s}\sim$ 10.6 GeV. The method uses correlated decays of pairs of tau leptons, each decaying to the $\pi \pi^0 \nu_{\tau}$ final state. The search is done within the framework of a model with a scalar boson exchange. In an analysis of a data sample corresponding to 12.2 million produced tau pairs collected with the CLEO detector, we find no evidence of violation of CP symmetry. We obtain a limit on the imaginary part of the coupling constant parameterizing the relative contribution of diagrams that would lead to CP violation to be $-0.046 <\Im(\Lambda) < 0.022$ at 90% C.L. This result provides a restriction on CP non-conservation in the tau lepton decays. As a cross check, we study the decay angular distribution and perform a model-independent search for a CP violation effect of a scalar exchange in single $\tau\to \pi\pi^0\nu_\tau$ decays. The limit on the imaginary part of the $\tau$ scalar coupling is $-0.033 < \Im(\Lambda) < 0.089$ at 90% C.L.

Strong coupling constant from τ decay within a renormalization scheme invariant treatment

We extract a numerical value for the strong coupling constant \alpha_s from the \tau-lepton decay rate into nonstrange particles. A new feature of our procedure is the explicit use of renormalization scheme invariance in analytical form in order to perform the actual analysis in a particular renormalization scheme. For the reference coupling constant in the \MSsch-scheme we obtain \alpha_s(M_\tau)= 0.3184 \pm 0.0060_{exp} which corresponds to \al_s(M_Z)= 0.1184 \pm 0.0007_{exp} \pm 0.0006_{hq mass}. This new numerical value is smaller than the standard value from $\tau$-data quoted in the literature and is closer to \al_s(M_Z)-values obtained from high energy experiments.

A study of the Lorentz structure in tau decays

This paper describes a measurement of the Michel parameters, \eta, \rho, \xi, \xi\delta, and the average \nu_{\tau} helicity, h_{\nu_{\tau}}, in tau lepton decays together with the first measurement of the tensor coupling in the weak charged current. The tau+tau- pairs were produced at the LEP e+e- collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the tau the measured values of the parameters were: \eta = -0.005 +/- 0.036 +/- 0.037, \rho = 0.775 +/- 0.023 +/- 0.020, \xi = 0.929 +/- 0.070 +/- 0.030, \xi\delta = 0.779 +/- 0.070 +/- 0.028, h_{\nu_{\tau}} = -0.997 +/- 0.027 +/- 0.011. The strength of the tensor coupling was measured to be \kappa^W_\tau = -0.029 +/- 0.036 +/- 0.018. The first error is statistical and the second error is systematic in all cases. The results are consistent with the V-A structure of the weak charged current in decays of the tau lepton.

Is there evidence for dimension-two corrections in QCD two-point functions?

The ALEPH data on the (non-strange) vector and axial-vector spectral functions, extracted from tau-lepton decays, is used in order to search for evidence for a dimension-two contribution, $C_{2 V,A}$, to the Operator Product Expansion (other than $d=2$ quark mass terms). This is done by means of a dimension-two Finite Energy Sum Rule, which relates QCD to the experimental hadronic information. The average $C_{2} \equiv (C_{2V} + C_{2A})/2$ is remarkably stable against variations in the continuum threshold, but depends rather strongly on $\Lambda_{QCD}$. Given the current wide spread in the values of $\Lambda_{QCD}$, as extracted from different experiments, we would conservatively conclude from our analysis that $C_{2}$ is consistent with zero.

Asymptotic structure of perturbative series for $\tau$ lepton observables

We analyze tau lepton decay observables, namely moments of the hadronic spectral density in the finite energy interval (0,M_\tau), within finite order perturbation theory including \alpha_s^4 corrections. The start of asymptotic growth of perturbation theory series is found at this order in a scheme invariant manner. We establish the ultimate accuracy of finite order perturbation theory predictions and discuss the construction of optimal observables.

Search for CP violation in tau lepton decay

Using the CLEO II detector a search for CP violation in τ lepton decay has been performed. CP violation can manifest itself as a difference in τ + and τ − angular distributions. Using this technique a sample of τ ± → K° π ± ν τ decays has been analyzed for evidence of CP violation. No evidence for CP violation is found and we set the model dependent 90% confidence limit −1.7 < gsin θ CP < 0.6.

Chiral sum rules and duality in QCD

The ALEPH data on the vector and axial-vector spectral functions, extracted from tau-lepton decays, is used in order to test local and global duality, as well as a set of four QCD chiral sum rules. These are the Das-Mathur-Okubo sum rule, the first and second Weinberg sum rules, and a relation for the electromagnetic pion mass difference. We find these sum rules to be poorly saturated, even when the upper limit in the dispersion integrals is as high as 3 GeV 2 . Since perturbative QCD, plus condensates, is expected to be valid for |q 2|≥ O(1 GeV 2) in the whole complex energy plane, except in the vicinity of the right hand cut, we propose a modified set of sum rules with weight factors that vanish at the end of the integration range on the real axis. These sum rules are found to be precociously saturated by the data to a remarkable extent. As a byproduct, we extract for the low energy renormalization constant L ̄ 10 the value −4 L ̄ 10=2.43×10 −2 , to be compared with the standard value −4 L ̄ 10=(2.73±0.12)×10 −2 . This in turn leads to a pion polarizability α E=3.7×10 −4 fm 3 .

First Search forCPViolation in Tau Lepton Decay

We have performed the first search for CP violation in tau lepton decay. CP violation in lepton decay does not occur in the minimal standard model but can occur in extensions such as the multi-Higgs doublet model. It appears as a characteristic difference between the τ− and τ+ decay angular distributions for the semileptonic decay modes such as τ−→K0π−ν. We define an observable asymmetry to exploit this and find no evidence for any CP violation.Received 21 May 1998DOI:https://doi.org/10.1103/PhysRevLett.81.3823©1998 American Physical Society

TheμτandeτDecays of Top Quark Pairs Produced inpp¯Collisions ats=1.8TeV

We present evidence for dilepton events from t tbar production with one electron or muon and one hadronically decaying tau lepton from the decay t tbar -> (l nu_l) (tau nu_tau) b bbar, (l=e, mu), using the Collider Detector at Fermilab (CDF). In a 109 pb^-1 data sample of p pbar collisions at sqrt(s) = 1.8 TeV we expect ~1 signal event and a total background of ~2 events; we observe 4 candidate events (2 e tau and 2 mu tau). Three of these events have jets identified as b candidates, compared to an estimated background of 0.28+-0.02 events.

Search for new mechanism of CP violation through tau decay and semileptonic decay of hadrons

If CP is violated in any decay process involving leptons it will signify the existense of a new force (called the X boson) responsible for CP violation that may be the key to understanding matter-antimatter asymmetry in the universe. We discuss the signatures of CP violation in (1) the decay of tau lepton, and (2) the semileptonic decay of $\pi$, K, D, B and t particles by measuring the polarization of the charged lepton in the decay. We discuss how the coupling constants and their phases of the coupling of the X boson to 9 quark vertices and 3 lepton vertices can be obtained through 12 decay processes.

Physics with taus at CDF

At high energy hadron colliders such as the Tevatron at Fermilab, the extremely high interaction rates and large background from hadronic jets make triggering on and reconstructing the decays of tau leptons to hadrons a major experimental challenge. CDF has developed techniques for tackling this problem, and this work has led to several new physics results presented herein, including a search for the charged Higgs, a search for third-generation leptoquarks, studies of the decay of the W boson to taus, and the tau decay modes of the top quark.

The hadronic current in tau lepton decay to two pseudoscalar mesons

I report on studies of the invariant mass spectra of the hadronic systems in the decays τ− → π−π0ντ and τ− → KS0π−ντ. These studies were performed using data obtained with the CLEO-II detector operating at the CESR e+e− collider. In addition to results from fits to phenomenological models, I present comparisons with data on e+e− → π+π− as a test of CVC.

Decays of tau leptons to final states containing K0S mesons.

Using data collected with the CLEO II detector at the Cornell Electron Storage Ring, we have studied the decays of tau leptons produced through e{sup +}e{sup -} annihilation into final states containing K{sub S}{sup 0} mesons, observed through their decays to {pi}{sup +}{pi}{sup -}. We present branching fractions for decays to five final states: {tau}{sup -} {yields} K{sup 0}h{sup -}{nu}{sub {tau}}, {tau}{sup -} {yields} K{sup 0}h{sup -}{pi}{sup 0}{nu}{sub {tau}}, {tau}{sup -} {yields} K{sup 0}K{sup -}{nu}{sub {tau}}, {tau}{sup -} {yields} K{sup 0}K{sup -}{pi}{sup 0}{nu}{sub {tau}}, and {tau}{sup -} {yields} K{sub S}{sup 0}K{sub S}{sup 0}h{sup -}{nu}{sub {tau}}, where K{sup 0}h{sup -} denotes the sum of the processes involving {bar K}{sup 0}{pi}{sup -} and K{sup 0}K{sup -} particle combinations. Substructure and mass spectra in these final states are also addressed.